Apparatus for towing arrays of geophysical devices

ABSTRACT

A ship is provided with booms that may be extended outwardly from the ship. Geophysical devices of different genera are towed from the booms, in the water behind the ship. A UHF antenna is located at the outboard tip of each boom. A line joining the two antennas defines a long reference base line for determining the absolute heading of the ship with respect to a meridian and to locate the absolute positions of selected towed geophysical devices with respect to two-dimensional space. The booms may be automatically oriented along a line perpendicular to the ship&#39;s track, independently of the ship&#39;s heading.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is concerned with towing arrays of geophysical devices ina body of water behind a specially-designed ship for the purpose ofconducting three-dimensional geophysical surveys such as might be foundin classes 367 and 181/0.5.

2. Discussion of the Prior Art

In seismic surveys at sea, a ship tows a suitable sound source such asan air gun (water guns, gas guns or sparkers may also be used). Atintervals of perhaps eight or ten seconds, the sound source is fired toproduce an acoustic wave having a spectrum in the 10-100 Hz (cycles persecond) range. The ship also tows an elongated array of hydrophones. Theacoustic waves generated by the sound source propagate through thewater, into the earth and are reflected by subbottom earth layers. Thereflected acoustic waves then return to the surface to be detected bythe hydrophones. The hydrophones convert the acoustic waves toelectrical signals which are sent to a signal utilization device forprocessing and archival storage. From the processed signals,geophysicists produce maps of the respective earth layers.

The energy output of a single sound source of the type hereinbeforelisted is relatively weak. In addition, certain types of sound sourcescreate objectionable wave forms such as bubble pulses or are deficientin certain frequency bands within the seismic spectrum. Therefore, theseismic sound sources are grouped in two- or three-dimensional tunedarrays, having a desired geometrical pattern, containing a large numberof sources to produce a more robust seismic spectrum.

The array dimensions parallel to the ship's track may be set up easilyenough by simply adjusting the lengths of the tow lines to therespective guns of the array. Perpendicular to the track, the lateraldimensions are sometimes established by use of paravanes such as shownby U.S. Pat. Nos. 4,130,078 and 4,525,813. That arrangement is notsatisfactory because the locations of towed paravanes are never knownaccurately. U.S. Pat. No. 4,038,630 teaches use of fixed booms oroutriggers mounted on each side of the stern of the ship to increase thelateral extent of the array. A somewhat more elaborate configuration isillustrated in U.S. Pat. No. 4,480,574. Here, several seismic soundsources are towed from pivotable booms on each side of the ship. Eachsound source is manipulated by an individual winch, there being severalwinches distributed along each boom. That teaching is not very practicalbecause first, the winch mechanisms are directly exposed to sea water,subjecting the winches to corrosion problems. Secondly because thewinches and cables are quite heavy, the number of winches that can besafely mounted on booms of reasonable size must be limited to avoidstructural failure of the booms. It is to be observed that the '630 andthe '574 patents teach deployment of geophysical devices of but a singlegenus, namely air guns.

In three-dimensional (3D) seismic exploration, the geophysical shipproceeds along lines of survey separated by some distance such as 80meters. A typical hydrophone array may be 3000 meters or more long andmay include 1000 or more hydrophones, all encased in a plastic jacket.Such a hydrophone array is commonly referred to as a streamer cable. Ifa single streamer cable is towed along a line of survey and insonifiedby an array of sound sources, reflection data from a single line ofprofile are acquired. If two or more streamer cables, towed from fixedpoints on the ship, laterally separated by the desired survey-linespacing, are towed behind the ship, then two or more lines of profilecan be surveyed at one pass of the ship, a decided economy of ship'stime and charges.

The beam of a typical geophysical survey ship rarely exceeds 10 to 15meters. If two streamer cables are towed directly behind each side ofthe ship, the cable separation is inadequate for dual-line surveying.Attempts have been made to increase the lateral separation of the cablesby use of paravanes. But because of the towing drag of a streamer cable,some 3000 to 6000 pounds (as compared with 100-200 pounds drag force forsound sources), paravanes of manageable size cannot achieve thenecessary lateral "lift" to hold the cables in position. Furthermore, ithas been found that some ships are unable to develop enough power to towthe paravanes at an acceptable velocity for economical survey work.

A long streamer cable under tow in the water is subject to erratic crosscurrents. The precise position of the cable is unknown. U.S. Pat. No.4,376,301 teaches use of transponders or pingers submerged below thewater surface, outboard of the ship, separated by 20 to 40 meters. Soundpulses radiated by the pingers are detected by special receivers securedalong the streamer cable at selected intervals. The travel time betweenthe pingers and the respective receivers is measured to calculate thereceiver positions relative to the ship by triangulation. Because of therelatively short base line, 20-40 meters, compared to the great lengthof the streamer cable, at least 3.0 km, that method is not veryaccurate. That problem is excaberated in view of errors in the ship'sheading which introduce a skew error in the reference base line betweenthe pingers. Heading errors are caused by the unknown gyro-compassprecession rate between in-port calibrations. Therefore the absolutelocation of the streamer cable in two-dimensional space isindeterminate.

Another cable-location technique is taught by U.S. Pat. No. 4,532,617.That method involves ranging on the streamer cable from two differentlocations involving use of a second slave ship. The cost of theadditional ship makes that practice, as taught by the '617 patent,unattractive.

At least one geophysical contractor advertises use of a tail buoysecured to the far end of the streamer cable. The tail buoy is equippedwith a navigational device and the head end of the streamer cable isheld in place by paravanes. Although the location of the tail of thestreamer cable may be known, the location of the head end is notaccurately known because of the paravane problem mentioned supra. Thatmethod is impractical.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a means for deploying anumber of geophysical devices of different kinds into a body of water,from a ship. It is a further object of this invention to provide meansfor determining the absolute position in two-dimensional space of theindividual geophysical devices.

In accordance with an aspect of this invention an elongated rotatableboom is mounted on each side of the stern of a ship. The booms may berotated from a stowed position on a deck of the ship to an extendedposition perpendicular to the ship's track. A first plurality ofgeophysical devices of a first kind, which may be seismic sound sourcesare suspended from the booms at spaced-apart intervals. A firsttraveling means is provided for manipulating selected ones of said firstplurality of devices. A second traveling means is provided to distributea second plurality of geophysical devices, which may be streamer cables,along each boom at other specified spaced-apart intervals.

In accordance with another aspect of this invention, a plurality ofindividually powered winches are mounted on the inboard end of eachboom. By means of interconnecting cables, each winch manipulates acorresponding geophysical device of the first kind such as a seismicsound source. The group of winches taken as a whole, mounted on theinboard end of each boom, serves as a counterweight for that boom.

In accordance with yet another aspect of this invention, anelectromagnetic navigational signal receiving means, such as an antenna,is mounted on the outboard end of each boom, for receiving navigationsignals thereby to determine accurately, the absolute position intwo-dimensional space of the ends of the booms. The separation of thetwo antennas is equal to at least 100 wavelengths of the navigationsignal. A third antenna may be mounted at the bow of the ship, therebyto accurately fix the position of the bow of the ship as well as theoutboard tips of the booms, in two-dimensional space. From those data,the absolute heading of the ship, relative to a meridian, may bedetermined.

In accordance with a further aspect of this invention, an acousticaldevice, such as a piezo electric pinger, is secured in the water at aknown position beneath the antenna at the tip of each boom. The pingersradiate acoustic pulses at a frequency in the range of 3.5 to 12.5 kHzor higher, well above the seismic frequency spectrum. the pingers areused to measure the ranges to the geophysical devices of a second genus,such as seismic streamer cables. From those measurements, the absoluteposition of the streamer cables can be determined realtive totwo-dimensional space. The separation of the pingers is equal to atleast 200 wavelengths of the radiated acoustic pulse.

In accordance with another aspect of this invention, the first pluralityof geophysical devices may include devices of genera other than seismicsound sources, such as one or more magnetometers or gas seep detectors.

In accordance with an aspect of this invention, the geophysical devicesof a second genus include receiver means for receiving acoustic signalsfrom the pingers.

BRIEF DESCRIPTION OF THE DRAWINGS

The benefits and objectives of this invention will be better understoodby reference to the appended detailed description and the drawingswherein:

FIG. 1 is a schematic plan view of a geophysical ship towing a pluralityof geophysical devices of different genera;

FIG. 2a is a schematic side view of the ship of FIG. 1;

FIG. 2b is a schematic stern view of the ship of FIG. 1;

FIG. 3 shows details of a typical boom;

FIG. 3a is a partial section ablong 3a--3a of FIG. 3;

FIG. 4 is a side view of a typical boom;

FIG. 5 is an end view along 5-5' of FIG. 3 of a typical boom;

FIG. 5a is a bottom view of carriage 120 showing construction details;

FIG. 5b is a view from below of trolley 102 showing constructiondetails;

FIG. 6 shows a means for maintaining the integrity of the geometry of asound source array relative to the ship's track; and

FIG. 7 shows a typical boom in the stowed position on a deck of theship.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIGS. 1, 2a and 2b, there are shown schematically plan,side and stern views of a ship 10 having booms 12 and 14, both shown inthe extended position, which are rotatable about pivots 16 and 18 fromthe extended position as shown to a stowed position on deck as shown bydashed arcs 17a and 17b. Note that the tow lines and geophysical devicesare not shown in FIG. 2b to avoid excessive complexity of the drawing.When extended, the booms may be guyed by lines 20 and 22 in aconventional manner. The booms, 12 and 14 have inboard portions 11a and11b relative to pivot 16 and 18. The booms also have outboard portions13a and 13b, including ends 15a and 15b, relative to pivots 16 and 18.The inboard portions 11a and 11b of booms 12 and 14 serve ascounterweights. The counterweights include a plurality of cable winchessuch as 23 and 25, to be described in more detail later. The outboardportion 13a of the boom 12 is hinged along a horizontal axis by hingepin 27a (FIG. 3) and may be elevated to a desired angle above ahorizontal plane by a hydraulic ram 29a that is interconnected betweenpivot 16 (FIG. 3) and the underside of the boom. Similarly for boom 14.The booms are about 40 meters long measured from the pivot points 16 and18. The booms are of closed box construction (see FIG. 5) rather thanopen truss construction and are buoyant with respect to the water. Theweight of a boom is about 18,000 pounds while the counterweight weighsmore than 10,000 pounds.

At the ends 15a, 15b of the outboard portions of the booms there aremounted electromagnetic navigation signal receiving means such asantennas 28a and 28b. The lengths of the booms are a multiple of thewavelength of the navigation signals. In a preferred embodiment of thisinvention, the frequency of the navigation signals preferably is in theUHF range of 420 to 450 MHz. Assuming a propagation velocity of 299,670km/sec over salt water, the signal wavelengths will range from 0.71 to0.67 meter. The separationof the antennas thus is more than 100wavelengths of the navigation signal. A line joining the two antennas,thus defines a long reference base line whose absolute orientation isknown in two-dimensional space. The addition of a third antenna 28c atthe bow of the ship provides a third absolute position reference. Thesignals from the three antennas may be combined in navigation computer46. Because the phase difference between the signals received at thethree antennas can be resolved to less than one cycle, the absoluteheading of the ship, relative to a meridian of longitude, can bedetermined to less than 30 minutes of arc. The long base line betweenantennas in terms of the wavelength of the navigation signal, permits aresolution in the absolute positioning of better than one part in ahundred.

A plurality of geophysical devices of different genera are towed inwater 30 behind ship 10. They are secured to booms 12 and 14 by suitabletow lines, air lines (not shown) and data-signal conductor lines (notshown). The devices of a first genus, include seismic sound sources suchas air guns 32a-f, devices of a second genus such as hydrophone streamercables 34a and 34b, mini-streamers 36a and 36b, and perhaps amagnetometer 38 and a gas seep detector 40. The tow lines to thegeophysical devices of the first genus are payed out or reeled in bymeans of the winches such as 23 and 25 Six winches and six devices areshown for each boom but up to twelve are preferred. The respectivegenera of geophysical devices are distributed along the booms atpre-selected spaced-apart intervals such as every two meters or otherspacings by first and second traveling means that will be described inmore detail later.

Piezo-electric acoustic devices or pingers of any commercial type, 42aand 42b are suspended beneath the ends of the outboard portions of thebooms 12 and 14, at a known position in water 30 relative to antennas28a and 28b. Acoustic devices 42a and 42b radiate acoustic pulses at aselected frequency in the range of 3.5 to more than 12.5 kHz,substantially above the seismic spectrum of 10 to 100 Hz, to avoidinterference therewith. At a water velocity of 1500 meters per second,the separation (80 meters) between the pingers is more than 200wavelengths. In operation, preferably every two or three seconds thepingers insonify appropriate receivers 44a and 44b mounted at theleading ends of streamer cables 34a and 34b. The transmission instant ofa pinger pulse is sent to seismic signal processor 45. The time ofreceipt of that pulse at the receivers 44a and 44b is transmitted backto processor 45 over suitable conductors (not shown). From the traveltime differences between emission of a pulse from pinger 42a to receiptof that pulse at receivers 44a and 44b, the ranges 48a and 48b to thereceivers are computed (dashed lines). Similarly the ranges 50a and 50bare computed from pinger 42b to receivers 44a and 44b. From thecross-triangulation results, the absolute positions of thestreamer-cable head ends are determined in two-dimensional space.

Although only two receivers 44a and 44b are shown in each streamer cable34a and 34b, it is to be understood that many more such receivers arepreferably distributed at spaced-apart intervals along the streamercables. Because the ship heading is accurately known, there is virtuallyno skew error in the orientation of the reference base line betweenantennas 28a and 28b and the corresponding pingers 42a and 42b. It is tobe understood of course, that the mini-streamers 36a and 36b could befitted with suitable receivers such as 44a and 44b. However, the need isless critical because the mini-streamers are relatively close to theship.

Pingers 42a and 42b are normally towed at a depth at least as great asthe depth of the air guns. The air bubble from the air guns interfereswith the propagation path of the pinger pulses. However because the airbubble rises to the water surface following firing of the air gun, byplacing the pingers at or below the gun level, interference does notoccur.

The seismic sound sources 32a-f, for simplicity hereinafter termed gunsor air guns by way of example but not by way of limitation, are trailedonly two or three hundred feet behind the ship and accordingly, theirposition is well known. It is primarily the positions of the head endsof the streamer cables that is critical because they stretch for severalkilometers behind the ship.

When not in use, the streamer cables are moved inboard along the boomsand are transferred by any convenient means to cable storage reels 52aand 52b. One reel may be mounted on the upper deck and the other may bemounted on a lower deck of the ship or both reels may be mounted on thesame deck. The traveling means for moving the streamer cable along thebooms will be described in detail below.

Of interest is the method for controlling the depth of the guns such as32f. The gun is secured to boom 14 by a tow cable 54 (FIG. 2a). A firingline 56 including trigger circuits, timing circuits and an air-supplyhose is arranged in a bundle such as taught by U.S. Pat. No. 4,525,813.Although the tow line is taut, an excess of firing-line slack is payedout so as to form a loop behind the gun due to water resistance when thegun is under tow. It turns out that the more excess firing line that ispayed out, the deeper the gun descends in the water for a given lengthof tow cable 54. By that means the gun depth can be controlled quiteaccurately. It is of course, to be understood that all underwatergeophysical devices are furnished with depth detectors of a conventionaltype which transmit the depth read-outs to the central seismic processor45. The depth of the streamer cables and mini-streamers are controlledby commercially-available hydroplanes or birds. The pingers 42a and 42bare held in a known position beneath antennas 28a and 28b by means ofV-fin depressors such as are made by Endeco Corp. of Marion, Ohio. Thedesign of the V-fin is such that the pingers are held nearly verticallybeneath the antennas 28a and 28b at normal ship speeds.

The remainder of this discussion will now center around the details ofthe first and second traveling means, the winch assemblies, and the boomrotation mechanism. Since both booms are identical we shall describeboom 12 as typical.

FIG. 3 is a side view of a typical boom assembly such as 12 (FIG. 1),showing construction details.

As previously stated, the boom consists of an inboard portion 11a and anoutboard portion 13a relative to pivot 16. The boom may be rotated froma stowed position on deck (FIG. 7) to an extended position (FIGS. 1 and2b) by a motor 60, preferably hydraulic, having a pinion 61 that mesheswith an internally-hobbed ring gear 62 shown in FIG. 3a. Pivot 16 isquite massive being about 1.5 meters in diameter. The outboard portionof boom 12 is hinged by a horizontally-disposed pin 27a. Hydraulic ram29a is provided to raise the outboard portion 13a of boom 12 to adesired angle above a horizontal plane. Although a single ram is shownin FIG. 3, three such rams are preferred.

The inboard portion 11a of boom 12, which also serves as acounterweight, includes a platform 64 upon which are mounted a pluralityof cable winches 23a-d. Four are shown in FIG. 3 but up to twelve arepreferred. Hydraulic power is applied to the winches individuallythrough lines 65a-d and control valves 66a-d that are coupled through amanifold 68 to a hydraulic supply 70 of any conventional type. Thewinches could of course, be electrically or pneumatically powered. A towcable 54 interconnects winch 23a with an air gun 74 through suitablesheaves 76 and 78. Air gun 74 is initially suspended from the outboardportion 13a of boom 12 by the tow cable 54. Winch 23a may then beactivated to lower the air gun in the water for towing behind the shipas shown in FIG. 1. Other cables 72a, 72b, 72c interconnect winches 23a,23c, 23d to other guns distributed along the boom. Cables to the moreremote guns may be threaded through suitable snatch blocks 80 to prevententanglement.

Assuming that the seismic sound sources are air guns, an air hose,electrical lines to trigger the guns, depthindicator read-out conductorsand other auxiliary data lines, are packed together in a firing linebundle as hereinbefore described. The firing line 56 passes from thedeck of the ship and over snatch blocks 84 and 86, along the outboardportion of the boom, over snatch blocks 88 and 90 and thence to gun 74.Firing line 56 is manipulated independently of tow cable 72 by means ofa hydraulically-driven cathead 92 of any conventional type. Firing line56 remains slack with respect to tow cable 72 and may be used to controlthe gun depth as shown in FIG. 2a. Firing lines to other guns arearranged similarly along the boom.

FIG. 4 illustrates in somewhat greater detail the method of attachmentof snatch blocks 76, 78 and 88, 90. Note that snatch blocks 76 and 88are suspended from hangers 94 and 96 that are welded to the boomstructure. As shown in FIG. 3, tow cable 54 is threaded through snatchblocks 76 and 78 while firing line 56 is threaded through snatch blocks88 and 90. Corresponding sets of snatch blocks are provided for otherguns along the boom.

FIG. 5 is a cross section of boom 12 along line 5-5' of FIG. 3.Referring now to FIGS. 4, 5, 5a and 5b the outboard portion 13a of boom12 supports an upper track 98 and a lower track 100. A first travelingmeans 102, such as a trolley rides along the lower track supported bythrust-rollers and support rollers 104a-f. Traveling means 102 includesan outwardly projecting, vee-shaped yoke 110 for engaging the tow cableof a selected air gun thereby to move the gun along boom 12 back towardsthe ship for servicing. It should be observed that yoke 110 will onlyengage the tow line of a gun that is suspended directly beneath theboom. For other guns that are under tow, the tow cables extend at anangle from the boom as shown in FIG. 2a and therefore are clear of theyoke.

First traveling means 102 is moved along the outboard portion 13a ofboom 12 by means of an endless cable or chain 112 that is driven by asuitable hydraulic motor 114 located near pivot 16. A channel 116provides return support for the cable or chain . The cable or chain 112is bolted to trolley 102 by suitable clamps 118. The cable or chain 112is conventionally supported at the outboard end 15a of the boom by anidler pulley (not shown).

A second traveling means or carriage 120 moves along upper track 98.Like trolley 102, carriage 120 is provided with thrust and supportrollers 122a-f. A padeye 124 extends outwardly from carriage 120 forreceiving a geophysical device of a second genus such as a seismicstreamer cable 34a. Carriage 120 is movable along the outboard portion13a of boom 12 by means of an endless chain or cable loop 128 that isactuated by hydraulic motor 130. Cable 128 is secured to carriage 120 bymeans of a remotely-controlled solenoid clamp 132. Solenoid clamp 132provides means for engaging or disengaging carriage 120 from actuatingcable 128. Thus, a first carriage such as 120 can be moved to a desiredposition along boom 12, whereupon it is disengaged from the cable. Othercarriages (not shown) can then be individually and selectively clampedto cable 128, moved into position and then disengaged. Thus, a pluralityof geophysical devices of a second genus can be distributed along theoutboard portion 13a of boom 12 at speced-apart intervals by use of asingle actuating cable. Carriage 120 includes a junction box 134 forreceiving electrical conductors 136 from seismic streamer cable 34a.Support hooks such as 138 distributed along the boom provide support forthe return portion of cable 128. The far end of cable 128 is supportedby an idler pulley at end 15a of boom 12 (not shown).

In geophysical surveying, it is necessary to arrange the guns in arrayshaving a selected geometrical pattern relative to the track of the shipover the sea floor. The purpose of an array is to enhance preferredfrequencies in the seismic spectrum, and more importantly, for purposesof beam steering. If the array geometry is skewed, beam-steering resultsbecome distorated. In fine weather, the ship's heading is substantiallythe same as the ship's track, absent severe sea currents. The arraygeometry is not skewed relative to the track. In bad weather and in theface of strong ocean currents, the ship must crab into the wind andcurrent to maintain the desired track. The crab angle may be quitelarge. If now, the booms 12 and 14 were constrained to remainperpendicular to the fore-and-aft axis of the ship as in the prior art,the geometry of the array of towed guns, relative to the ship's actualtrack over the sea floor, would be severely distorted.

FIG. 6 shows ship 10 headed into the wind at a considerable angle to thedesired track. An array of guns (triangles) such as 32a-f of FIG. 1, istowed behind the ship. Booms 12 and 14 are rotated about pivots 16 and18 so as to remain perpendicular to the ship's track independently ofthe heading of the ship. Because of the data provided by the threeantennas 28a-c, the ship's absolute heading, as well as its absoluteposition in two-dimensional space, is known very accurately. Navigationcomputer 46 processes the navigation signals so as to provideinformation to a servo system of any desired type (not shown) to causemotor 60 (FIG. 3) to automatically swing the corresponding boom to adirection perpendicular to the ship's actual track.

Pivots 16 and 18 are fixedly mounted on each side of the stern of theship, separated laterally by about 8 meters. Assuming a crab angle ofabout 25 degrees, the linear displacement of the pivots relative to theship's track is about 3.5 meters. That displacement is relativelytrivial compared to the 40-meter diagonal skew of the devices towedbehind the ship had the booms been constrained to remain perpendicularto the ship's heading. Furthermore, the integrity of the prescribedsurvey-line spacing is preserved.

FIG. 7 shows the ship 10 with the booms in the stowed position on deck.When stowed, the boom such as 14 rests on a cradle such as 140. Theguns, such as 32, are suspended from the booms and may rest on deck, asshown, for servicing.

In operation, the booms initially are in the stowed position as shown inFIG. 7. The air guns are suspended directly beneath the booms by theircorresponding tow cables. The firing line remains slack. Uponcommencement of a survey, the booms are elevated by the hydraulic ramsuntil the suspended air guns clear the deck rail 142 of ship 10. Thebooms are then swung outwardly around pivots 16 and 18 as shown inFIG. 1. The geophysical devices of a second genus such as the seismicstreamer cables 34a and 34b are payed out from their respective cablestorage reels 52a and 52b and are then transferred to correspondingsecond traveling means such as a carriage 120. The streamer cables arethen distributed along the outboard portions of the booms atspaced-apart intervals. Only one streamer cable is shown for each boom,but more than one streamer cable may be deployed. It should be observedin FIGS. 3, 4, and 5, that a guard rail 127 is secured between thehangers along the boom such as 94 and 96 to prevent the streamer cablefrom becoming entangled therewith.

After the streamer cables have been deployed and distributed, thesuspended air guns are lowered into the water behind the ship from theirrespective tow cables by means of their corresponding winches such as23a-d. The air guns may be towed at different distances behind the shipto form an array of desired geometry. Of course, as each gun is loweredaway, its corresponding firing line is payed out over cathead 92. Oncein the water, each gun is adjusted to a desired depth by paying outfiring-line slack as shown in FIG. 2a.

During operation, a gun malfunction may occur. In that event, thedefective gun is reeled in by the corresponding winch along with itsfiring line. At the same time, first traveling means, trolley 102, ismoved along the boom outwardly away from the ship until the trolley isbeyond the appropriate snatch block hanger such as 94, FIG. 5. When thegun is out of the water and hanging vertically beneath the boom on itstow cable, trolley 102 is moved along the boom towards the ship. Yoke110 then engages the tow cable such as 54 to manipulate the defectivegun back aboard the ship. Of course, during that process, the tow cableand firing line are slacked-off as required.

Once the ship is underway, the navigation system 46 tracks the positionsof the seismic streamer cables and provides ship's heading information.The heading information is continuously compared with the ship's gyrocompass to correct for precession error. The navigation system alsoprovides correction information to a suitable servo system forautomatically maintaining the proper orientation of the booms relativeto the ship's track independently of the heading of the ship.

Those skilled in the art will doubtless conceive of variations in thedesign and operation of this invention, but which will fall within thescope and spirit of this disclosure which is limited only by theappended claims.

We claim as our invention:
 1. An arrangement for deploying a pluralityof geophysical devices of different genera in a body of water,comprising:a ship having a bow and a stern for towing said geophysicaldevices; an elongated boom pivotally mounted on each side of the sternof the ship, means for rotating each said boom from a stowed position ondeck to an outboard extended position, each said boom having an inboardportion and a horizontally-hinged outboard portion; means, mounted onthe inboard portion of each said boom for counterweighting therespective booms; a first plurality of geophysical devices of a firstgenus suspended from each said boom for towing in the water behind theship when said booms are in the extended position; a trolley means,movable along the outboard portion of each said boom for handlingselected ones of the first plurality of geophysical devices when thebooms are in the extended position; at least one carriage means movablealong the outboard portion of said booms for distributing a secondplurality of geophysical devices of a second genus at spaced apartintervals along the outboard portion of the booms and for towing thesecond plurality of geophysical devices through the water behind theship when the booms are in the extended position; and means mounted onthe end of the outboard portion of each said boom for receivingelectromagnetic navigation signals having a preselected frequency, aline joining the signal-receiving means defining a long reference baseline for determining the absolute positions in twodimensional space, ofsaid pluralities of geophysical devices.
 2. The arrangement as definedby claim 1 in which:the separation between the navigation-signalreceiving means, when the booms are extended, is at least 100wavelengths of said electromagnetic navigation signals.
 3. Thearrangement as defined by claim 1, further comprising:acoustic meanssuspended in the water from the ends of the outboard portions of thebooms, when the booms are in the extended position, at a known locationbeneath the navigation signal receiving means, for acoustically rangingon said second plurality of geophysical devices to establish theabsolute positions of respective ones of the second plurality ofgeophysical devices in two-dimensional space.
 4. The arrangement asdefined by claim 3, in which:the acoustic devices emit acoustic signalsinto the water at a frequency substantially above the normal seismicfrequency spectrum; and the separation between said acoustic devices ismore than 200 wavelengths of said acoustic signals.
 5. The arrangementas defined by claim 1 in which:said first genus of geophysical devicesincludes seismic sound sources.
 6. The arrangement as defined by claim 1in which:said second genus of geophysical devices includes seismicstreamer cables.
 7. The arrangement as defined by claim 1, furthercomprising:a third means mounted near the bow of said ship for receivingelectromagnetic navigation signals; means for combining theelectromagnetic navigation signals received by said third signalreceiving means with the navigation signals received by the signalreceiving means mounted on the ends of the outboard portions of eachsaid boom, thereby to measure the absolute heading of the ship withrespect to a meridian.
 8. The arrangement as defined by claim 1, furthercomprising:a plurality of winches mounted on the inboard portions of thebooms; a plurality of tow cables interconnected between each one of saidwinches and a corresponding one of said plurality of geophysical devicesof the first genus; and means for applying power individually toselected ones of said plurality of winches for deploying into the waterat known preselected positions behind the ship, and for recovering,corresponding ones of said plurality of devices of the first genus. 9.The arrangement as defined by claim 8, in which:the plurality of winchestaken together as a group and as mounted on the inboard portion of eachsaid boom, constitute counterweights for the booms.
 10. The arrangementas defined by claim 1, further comprising:means for adjusting the boomsto a preselected angle above a horizontal plane.
 11. The arrangement asdefined by claim 1, wherein each said boom is buoyant with respect tothe water.
 12. The arrangement as defined by claim 1, in which:theplurality of devices of the first genus includes at least onemagnetometer.
 13. The arrangement as defined by claim 1, in which:theplurality of geophysical devices of the first genus includes at leastone gas seep detector.
 14. An arrangement for towing a plurality ofgeophysical devices of different genera through a body of watercomprising:a ship; a pivotally-mounted horizontally-hinged boom on eachside of said ship, extending outwardly therefrom; means forcounterweighting the outwardly extended booms; means for towing a firstplurality of geophysical devices of a first genus through the water atfirst spaced apart intervals from said booms; a trolley means forretrieving from the water a selected one of said plurality ofgeophysical devices of said first genus; at least one carriage means,movable along said booms for distributing a second plurality ofgeophysical devices of a second genus in the water at second spacedapart intervals; and antenna means, mounted on the outboard ends of thebooms for receiving electromagnetic navigation signals, a line joiningsaid antennas defining a reference base line that is long relative tothe wavelength of the navigation signals for determining the absolutepositions in two-dimensional space of the towed geophysical devices. 15.The arrangement as defined by claim 14, further comprising:acousticmeans, suspended from the ends of the booms at a known position beneathsaid antennas, for transmitting acoustic pulses at a frequencysubstantially above the seismic signal frequency range; pulse receivingmeans associated with the geophysical devices of said second genus; andmeans on said ship interconnected with said acoustic means and saidpulse receiving means for measuring the elapsed time betweentransmission of pulses from said acoustic means and receipt of saidtransmitted pulses by the pulse receiving means, thereby to calculate bytriangulation, the absolute positions of said pulse receiving means intwodimensional space.
 16. The arrangement as defined by claim 1, furthercomprising:means for automatically orienting each said boom in adirection perpendicular to the ship's track independently of the ship'sheading.
 17. The arrangement as defined by claim 8, further comprising:afiring line coupled to each said geophysical device of the first genus;means for applying sufficient slack to said firing line relative to thecorresponding tow cable, to adjust the depth in the water of each saidgeophysical device of the first genus.
 18. The arrangement as defined byclaim 14, further comprising:means for automatically orienting each boomin a direction perpendicular to the ship's track independently of theship's heading.
 19. An arrangement for towing a plurality of geophysicaldevices of different genera through a body of water, comprising:a ship;a pivotally-mounted horizontally-hinged boom on each side of said ship,extending outwardly therfrom; means for counterweighting the outwardlyextending booms; means for towing a first plurality of geophysicaldevices of a first genus through the water at first spaced apartintervals from said booms; a trolley means for retrieving from the watera selected one of said plurality of geophysical devices of said firstgenus; at least one carriage means, movable along said booms fordistributing a second plurality of geophysical devices of a second genusin the water at second spaced apart intervals; and means forautomatically orienting each boom in a direction perpendicular to theship's track independently of the ship's heading.